1. Field of the Invention
The present invention relates to an optical scanning system for a laser printer, or the like.
2. Description of the Related Art
In typical optical scanning systems of the related art, a polygonal mirror, which deflects light emitted from a light source; and a scanning lens, an f.theta. lens which converges the light reflected by the polygonal mirror onto an image surface to scan the same at a constant speed; are provided.
However, in general, the f.theta. lens is made of a plurality of lenses of a complex shape having toric lens surfaces, hence being difficult to accurately produce the f.theta. lens. If a lens made of a plastic material which can be easily machined is used, the plastic material must have good transparent properties, hence, limiting the selection of materials which can be used. Consequently, a reflection type optical scanning system has been recently proposed in which a reflection mirror with a curved surface is employed for constant speed scanning, in place of an f.theta. lens.
In an optical system using a curved reflector (a curved mirror), it is necessary to provide a half mirror to split incident light from a deflector and emission light from the curved reflector. However, the quantity of light which can be transmitted is reduced by the half mirror. Moreover, in the optical system, the curved reflector must be inclined with respect to the optical axis in the sub-scanning direction. Consequently, a bow (curvature of the scanning lines) is caused.
To eliminate the bow which is caused by the inclination of the curved reflector in the sub-scanning direction, the light is made incident upon the deflector at a predetermined incident angle in the sub-scanning direction.
Recently, a multiple beam scanning system has been proposed in which beams which are spaced in the main scanning direction and the sub-scanning direction are simultaneously moved to carry out a high-speed drawing operation. In such a multiple beam scanning system, it is preferable that the distance between the scanning lines be kept constant. However, there is a possibility that the distance between the scanning lines will vary depending on the image height in the sub-scanning direction. The difference in the curvature between the scanning lines is referred to as a differential bow. The differential bow leads to an irregular or uneven drawing or reduces the drawing efficiency. The differential bow is produced, for example, when the magnification in the sub-scanning direction varies depending on the image height in the main-scanning direction.
A reflection type optical scanning system is disclosed in, for example, Japanese Unexamined Patent Publication No. 8-68957. In this scanning system, the curved reflector is inclined in the sub-scanning direction with respect to the optical axis to split the incident light and the emission light. The light is made incident upon a polygonal mirror at a predetermined incident angle in the sub-scanning direction, so that the curvature of the scanning line caused by the curved reflector can be compensated. Other aberrations can be also effectively corrected. However, the f-number in the sub-scanning direction decreases as the image height in the main scanning direction increases. Consequently, the spot diameter on the surface to be scanned varies and a differential bow is still produced.
In order to eliminate the differential bow produced due to the curved reflector being inclined in the sub-scanning direction, the light rays are made incident on the curved reflector at an incline in the sub-scanning direction. Moreover, due to the correction of surface-tilt of the deflector, the incident light rays become image lines. The scanning line of the light rays reflected from the deflector bend in the sub-scanning direction. Since the image line also inclines, distortion of the wave surface (skew distortion) of the light rays occurs.